The sun is a bright object of which the observation with high resolution is of interest. Telescopes are used for this purpose. An inadmissible heating of the secondary optics occurs in telescopes with large numerical aperture (=small f number). A compact and stable construction is the reason to use a large numerical aperture.
A known method to reduce the incoming amount of light is a semi transparent glass plate. The semi transparency is achieved by an absorbing material added to the glass melt or by an absorbing and/or reflecting coating, which is evaporated on the glass surface. Both sides of the glass plate have to be worked with very high precision because otherwise the resolution is reduced by optical aberrations. This makes the semi transparent glass plate an expensive piece of the telescope. For large aperture diameters (&gt;0.8 m) it is even doubtful whether the realisation is technically feasible.
A screen, which simply reduces the diameter of the aperture also reduces the resolution limit of the telescope, because the physical resolution limit is proportional to the wavelength of the light divided by the diameter of the aperture.
The obscuration of the centre part of the aperture gives an annular aperture. In books on optics it is claimed, that the resolution limit of an annular aperture is even slightly better than for a circular aperture of the same diameter. However, this is only true for the separation of two point sources. The image of structures on a light background has an inadmissible reduction of the contrast by the diffraction light.
The invention provides a screen, which reduces the incoming light with preservation of the resolution also of structures on a light background.